Data processing: measuring – calibrating – or testing – Measurement system in a specific environment
Reexamination Certificate
1999-10-19
2003-01-28
Lefkowitz, Edward (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
C367S159000
Reexamination Certificate
active
06512980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Background of the Invention
The present invention relates generally to marine seismic surveying and, more particularly, to a method and apparatus for reducing the signal noise from vertical movement in a dual sensor towed streamer cable caused by vibrations in the stress members of the streamer.
2. Description of Related Art
Seismic surveying is a method for exploring subterranean formation layers in the earth. An acoustic source generates seismic waves, which insonify the formation layers. Differences in acoustic impedance of adjacent formation layers cause a portion of the seismic waves to reflect from the interfaces between the formation layers. Acoustic impedance varies across formation layers since it is the product of seismic wave velocity and rock density. Seismic sensors detect the seismic waves reflected upward from the formation interfaces and record wave amplitude versus time of arrival as electrical signals for later analysis regarding the locations of the formation interfaces.
Marine seismic surveying is seismic surveying for formation layers in parts of the earth located beneath bodies of water. An acoustic source placed in the water, such as an airgun, generates the seismic waves which insonify the subterranean formation layers. Seismic sensors, typically arrayed at intervals along a streamer cable towed in the water behind a vessel, detect the reflected seismic waves. Marine seismic surveying typically uses pressure sensors, such as hydrophones, to detect changes in water pressure caused by seismic compression and rarefaction waves propagating through the water. The pressure sensors detect the primary pressure waves traveling upward in the water after reflection from the formation interfaces in the earth below the water. The pressure sensors also detect secondary pressure waves traveling downward in the water after a portion of the primary waves traveling upward reflect down from the water surface above. The air-water interface at the water surface has a large contrast in acoustic impedance which causes a large downward reflection. Secondary reflections are unwanted ghost waves, a type of noise in the seismic signal.
The water-earth interface at the water bottom may also have a large contrast in acoustic impedance. Thus, the downward-traveling secondary reflections from the water surface may reflect back upward again from the water bottom. Thus secondary reflections may continue to reverberate through the water column from surface to bottom and back. Water column reverberation is a serious source of signal noise obscuring the primary reflections carrying the sought-after information concerning the subsurface formation layers.
FIG. 1
shows a diagrammatic view of marine seismic surveying employing a seismic streamer cable, generally designated as
100
. A ship
102
tows a seismic streamer
104
through a body of water
106
. The seismic streamer
104
contains a plurality of sensors
108
. Subterranean substrata, such as
110
and
112
, to be explored, are located in the earth
114
beneath the body of water
106
. Interfaces, such as
116
, separate the substrata. An acoustic source
118
, such as an air gun, creates seismic waves in the water
106
. A portion of the seismic waves travel downward along ray paths
120
through the water
106
toward the earth
114
. A portion of the downward-traveling seismic waves reflect upward from an interface, such as interface
116
between substrata
110
and
112
. The reflected, upward-traveling seismic waves are primary reflections from the formation layers. The primary reflections travel upward along ray paths
122
, a portion of which intersect the towed streamer
104
. Sensors
108
deployed in the towed streamer
104
detect the primary reflections. The primary reflections travel past the towed streamer
104
and continue along ray paths
122
upward toward the air-water interface
124
at the surface of the body of water
106
. A portion of the seismic waves comprising the primary reflections reflect downward from the air-water interface
124
. The twice-reflected, downward-traveling seismic waves are secondary reflections from the water surface. The secondary reflections travel downward along ray paths
126
, a portion of which intersect the towed streamer
104
. The sensors
108
deployed in the towed streamer
104
detect the secondary reflections from the air-water interface
124
.
The towed streamer
104
contains a plurality of sensors
108
. Towed streamers
104
typically carry pressure sensors, such as hydrophones, which will be described below in FIG.
2
. Dual sensor towed streamers
104
carry pairs of pressure sensors and motion sensors, such as geophones or accelerometers. The present invention adds a third sensor, a noise reference sensor, which will be described below in FIG.
3
. The third sensor is a variant of the prior art pressure sensor.
FIG. 2
shows a diagram of a pressure sensor
200
, an acceleration-canceling hydrophone, typically used in a towed streamer. The pressure sensor
200
typically comprises a housing
202
having a first end and a second end, a first element
204
mounted at the first end of the housing
202
, and a second element
206
mounted at the second end of the housing
202
. The first element
204
is mounted parallel to the second element
206
. The housing
202
is typically made of brass and the first and second elements
204
,
206
are typically made of piezoelectric crystal. A first pair of electric wires
208
,
210
attaches to the opposing faces of the first element
204
and a second pair of electric wires
212
,
214
attaches to the opposing faces of the second element
206
. The arrows in
FIG. 2
show the relative polarities of the connections.
FIGS. 3
a
-
3
d
show conceptual diagrams of an acceleration canceling hydrophone
300
subject to accelerations and passing seismic waves. The electric wires
308
,
310
,
312
and
314
are connected so that flexures of the elements
304
,
306
such as shown in
FIGS. 3
a
and
3
b
generate output voltages which add, resulting in a nonzero signal. The pressure manifestation of a compression seismic wave propagating past the pressure sensor
300
causes the flexure of the elements
304
,
306
shown in
FIG. 3
a
, while the pressure manifestation of a rarefaction seismic wave propagating past the pressure sensor
300
causes the flexure of the elements
304
,
306
shown in
FIG. 3
b
. This flexures of the elements
304
,
306
as shown in
FIGS. 3
c
and
3
d
generate output voltages which cancel, resulting in a substantially zero signal. Upward motion of the pressure sensor
300
through a fluid causes the flexure of the elements
304
and
306
shown in
FIG. 3
d
. The above-described con figuration accomplishes the acceleration canceling property of the typical marine hydrophone
300
. The polarity indications on FIGS.
3
(
a
)-
3
(
d
) show the relative polarities of signals generated for the flexure of elements
304
,
306
as shown.
Marine seismic surveying also uses motion sensors, detecting particle velocity or acceleration, in addition to pressure sensors. Motion sensors typically used in marine seismic surveying are geophones and accelerometers. Motion sensors detect the vertical velocity or acceleration of water particles accompanying seismic waves propagating past the sensors. Thus motion sensors detect primary and secondary reflections, just as pressure sensors do. Proper combination of the signals from pressure sensors and motion sensors can lead to a reduction of secondary reflections from the water surface in the seismic signal. The air-water interface causes a reverse in polarity in the downward reflected pressure wave, since the acoustic impedance of the water exceeds the acoustic impedance of the air. Thus pressure sensors detect a reverse in phase polarity for the secondary reflections from the water surface. The air-water interface does not cause a reverse in polarity in the vertical motion wave. Thus motion sensors do not sense a reverse in phase polari
Figatner David S.
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Taylor Victor J.
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